Resistance material and electrical resistor made therefrom

ABSTRACT

A RESISTANCE MATERIAL INCLUDING A MIXTURE OF A GLASS FRIT AND FINELY DIVIDED PARTICLES OF TANTALUM CARBIDE AND TITANIUM. THE RESISTANCE MATERIAL IS ADAPTED TO BE APPLIED TO AND FIRED ON A SUBSTRATE TO FORM AN ELECTRICAL RESISTOR WHICH WILL READILY FUSE OR OPEN WHEN SUBJECTED TO AN OVERLOAD TO PREVENT EXCESSIVE OVERHEATING OF THE RESISTOR.

Jan. 29, 1974 G. D. M KENZlE 3,788,997

RESISTANCE MATERIAL AND ELECTRICAL RESISTOR MADE THEREFROM 20 7/ TA N/UMoxms PARTICLES l4-RES/STANCEL 1 7 MATERIAL Filed Dec. 17, 1971 I8-TANTALUM CARBIDE PARTICLES l6- GLASS MATRIX INVENITOR GEORGE 0. MACKENZ/E gMXMM ATTOR/VE'Y "United States Patent RESISTANCE MATERIAL ANDELECTRICAL RESISTOR MADE THEREFROM George D. MacKenzie, Maple Glen, Pa.,assignor to TRW, Inc., Cleveland, Ohio Filed Dec. 17, 1971, Ser. No.209,139 Int. Cl. B44d 1/02; H01b 5/04 US. Cl. 252-635 8 Claims ABSTRACTOF THE DISCLOSURE A resistance material including a mixture of a glassfrit and finely divided particles of tantalum carbide and titanium. Theresistance material is adapted to be applied to and fired on a substrateto form an electrical resistor which will readily fuse or open whensubjected to an overload to prevent excessive overheating of theresistor.

SPECIFICATION comprises a mixture of a glassfrit and finely dividedparticles of a conductive material. The mixture is applied to the,surface of a ceramic substrate and fired. When cooled, there isprovideda film ofglass having the particles of the conductive materialdispersed throughout the glass film. To provide a resistor of a desiredresistance value, the resistance material film is generally in the formof an elongated, narrow path. Also, the resistance material film isgenerally coated with a plastic material to protect it from mechanicaldamage and from. moisture and other contaminates in the atmosphere.

, In the use of such vitreous enamel resistors a problem has arisen whenthe resistors are subjected to overload conditions. All resistors,including those made with vitreous enamel resistance materials, heat upwhen placed under a load in use. The higher the'load the hotter. theresistors will become. It has been found that resistors 'made with thevitreous enamel resistance materials when subjected to overloadconditions heat up to such an extent that they can cause the protectivecoatings or other flammable material near the resistor to catch fire. Toprevent i such a dangerous condition from occurring, it would be sistormade therefrom'which can be made over a wide range-of resistance-valueswith stable resistance characteristics and which will not excessivelyoverheat when subjected to an overload condition.

It is a furtherobject ofthe present invention to provide a vitreousenamel resistancematerial and aresistor made therefrom which will fuseor open to a high resistance condition when subjected to an overloadcondition, so as to prevent excessive overheating of the resistor.

Other objects will appear hereinafter.

The invention accordingly comprises a composition of matter and theproduct formed therewith possessing the characteristics, properties andrelation of constituents which will be exemplified in the compositionhereinafter described, and the scope of the invention will be indicatedin the claims.

The drawing is a cross-sectional view, on a highly exaggerated scale, ofa portion of a resistor made from the resistance material of the presentinvention.

In general, the vitreous enamel resistance material of the presentinvention comprises a mixture of a glass frit and finely dividedparticles of a mixture of tantalum carbide (TaC) and titanium (Ti). Byfinely divided particles it is meant an average particle size of notgreater than about 5 microns. The mixture of the tantalum carbide andtitanium is present in the resistance material in the amount of 10% toby weight. The ratio of the tantalum carbide to titanium in theresistance material may be varied depending on the resistivity andtemperature coefiicient of resistance desired for the resistancematerial. Increasing the amount of the tantalum carbide with respect tothe amount of titanium inthe resistance material will increase theresistivity and will make the temperature coeflicient of resistance morepositive. Thus, by adjusting the ratio of the amount of tantalum carbideto the amount of titanium and varying the amount of the conductivemixture in the resistance material, a desired temperature coefiicient ofresistance canbe obtained at a desired resistivity of the resistancematerial.

The glass frit used in the resistance material of the present inventionmay be of any well known composition which has a softening point belowthat of the tantalum carbide and titanium. The glass frits preferablyused are the borosilicate frits, such as lead borosilicate frit, bismuth, cadmium, barium, calcium or other alkaline earth borosilicatefrits. The preparation of such glass frits is well known and consists,for example, of melting together the constituents of the glass in theform of the oxides of the constituents, and pouring such moltencomposition into water to form the frit. The batch ingredients may, ofcourse, be any compound that will yield the desired oxides under theusual conditions of frit production. For example, boric oxide will beobtained from boric aicd, barium oxide will be produced from bariumcarbonate, etc. The coarse frit is preferably milled in a ball mill withwater to reduce the particle size of the frit andto obtain'a'frit'bf'subs'tantially uniform size.

To make the resistance material of the present invention, commerciallyavailable tantalum carbide and titanium are each pre-milled to produceeach of those materials in'p'owdered form of the desired particle size.The tantalum carbide, titanium and glass frit in the desiredproportionsto achieve a resistance material of the desired resistivityand temperature coefiicient of resistance are then throughly mixedtogether, such as by ball milling in water or an organic medium such asbutyl carbitol acetate. After the ingredients are blended, the viscosityof the mixture-is adjusted such as by removing or adding the liquidmedium, to the proper viscosity for the desired manner of. applying theresistance material to make a resistor.

To make a resistor with the resistance material of the presentinvention, the resistance material is applied in a uniform thickness onthe surface of a ceramic body. The ceramic body may be comprised of anyceramic material which can withstand the firing temperature of theresistance material composition. For example, the ceramic body may beglass, alumina, steatite, porcelain, barium titanate or the like. Theresistance material may be coated on the ceramic body by brushing,dipping, spraying or screen stencil application. The ceramic body andresistance material coating is then fired in a conventional furnace at atemperature at which the glass frit softens but less than the meltingtemperature of the tantalum carbide and titanium. The resistancematerial is preferably fired in a nonoxidizing atmosphere, such asargon, helium or nitrogen. When the ceramic body and resistance materialare cooled, the vitreous enamel hardens to bond the resistance materialto the ceramic body. When the resistance material is fired, the titaniumreacts with the glass of the frit to form semiconducting suboxides oftitanium, TiOx. Thus, the conducting particles in the fired resistancematerial are tantalum carbide and the titanium oxide. In many cases, thetitanium particles are not completely converted to the suboxides andrelic titanium structures remain in the material. The starting materialcan also include the suboxides of titanium in place of part or all ofthe titanium metal to produce the resistors with the desiredcharacteristics.

Referring to the drawing, there is shown a resistor, generallydesignated as 10, made from the resistance material of the presentinvention. Resistor 10 comprises a ceramic body 12 having a layer 14 ofthe resistance material coated on a surface thereof. The resistancematerial layer 14 comprises a glass matrix 16 having the tantalumcarbide particles 18 and the TiO particles 20 embedded in and dispersedthroughout the glass matrix 16.

Table I shows, the electrical characteristics, i.e. resistance andtemperature coeflicient of resistance, of a number of resistors of thepresent invention having various total amounts of the tantalum carbideand titanium and various ratios of the tantalum carbide to titanium. Theresistors were made in the manner described above, and the firingtemperature and time are also shown in Table I.

TABLE I Temp. coefi. of Weight Firing conditions resistance, percentResistp.p.m./ O. of TaG TaC/Ti Temp., Time, ance, and Ti (volume) C.min. fl/El 55 C. +150 0.

1/1 1, 150 30 89 682 507 1/1 1, 100 30 20. 5K 1, 215 846 1/1 1, 100 305. M 4, 233 2, 110 2/1 1, 150 60 5. +150 +130 2/1 1, 100 14 +73 +66 2/11, 150 30 6 +89 +83 1/1 1, 100 30 100 76 1/1 1, 150 30 9 ----144 -1122/1 1, 100 30 56 i154 2. 5/1 1, 100 30 100 =l=30 :|:23 3/1 1, 100 30 4953:16 :|=9

From Table I it appears that the optimum range of tantalum carbide andtitanium is between 45% and 70% by weight of the composition forproducing minimized temperature coefiicient of resistancecharacteristics for the resistors.

Table II shows the electrical characteristics of 'a miniber of resistorsof the present invention made with glass frits of diiferentcompositions. In each of these resistors the total amount of tantalumcarbide and titanium is 67% TABLE II Firing conditions Temp. coefl. ofResistrelistance Temp. Time, ance, Glass frit o 01 min. sz n 0. +15o 0.

Ma nesimn borosilicite 1, 100 30 14 +73 +66 28 13 E23 E22 b silicate--.1, 100 b d l f f? 1, 150 30 18 -174 188 Banum boroalumk 1, 100 30 24-718 459 1, 150 37 817 581 The resistors of the present invention havevery good flammability characteristics in that they do not excessivelyoverheat when subjected to an overload. This good flammabilitycharacteristic is the result of the resistors quickly converting to ahigh resistance condition when subjected to an overload. For example,the resistors will fuse to an open circuit condition withinapproximately two seconds when subjected to 40 watts, and the fusingcondition occurs more quickly as the wattage increases. Thus, there isprovided by the present invention a vitreous enamel resistance materialand resistor made therefrom which can be provided over a wide range ofresistance values, is relatively stable with regard to temperature, andhas good flammability characteristics so that it'will not catch firewhen subjected to an overload.

It should be understood that the examples of the resistors andresistance materials of the present invention shown in Tables I and IIare given merely to illustrate certain details of the invention, and arenot to be taken as in any way limiting the invention thereto; Thepresent invention may be embodied in other specific forms Withoutdeparting from the spirit or essential attributes thereof, and,accordingly, reference should be made to the append ing claims, ratherthan to the foregoing specification,'as indicating the scope of theinvention.

I claim: I

1. A vitreous enamel resistance composition adapted to be applied'to andfired on a substrate to form an electri: cal resistor consistingessentially of a glass frit and finely divided particles of tantalumcarbide and particles containing titanium, the tantalum carbide andparticles containing titanium being present in'the amount of between byweight and the ratio of tantalum carbide to titanium is 21:1 by volume.The firing conditions are shown in Table I 10% and 75% by weight. I

2. A vitreous enamel resistance composition in accordance with claim 1in which the particles containing titanium include particles of an oxideof titanium.

3. A vitreous enamel resistance composition in accordance with claim 1in which the tantalum carbide and titanium are present in the amount ofbetween 45% and 70% by weight.

4. A vitreous enamel resistance composition in accordance with claim 3in which the particles containing titanium include particles of an oxideof titanium.

5. An electrical resistor having good flammability characteristicscomprising a ceramic body having on a surface thereof a coating of avitreous enamel resistor composition consisting essentially of finelydivided particles of tantalum carbide and particles containing titaniumembedded in 'aglass matrix the tantalum carbide and titanium containingparticles being present in the amount of between 10% and 75% by weight.

6. An electrical resistor in accordance with claim 5 in which .theparticles containing titanium include particles of an oxide of titanium.I

7. An electrical resistor in accordance with claim 5 in which thetantalum carbide and titanium are present in the resistor composition inthe amount of between 45% and 70% by weigh 8. An electrical resistor inaccordance with claim 7 2,693,521 11/1954 Alexander 117--22 in which theparticles containing titanium include particles 3,277,020 10/1966 Rao252-51 of an oxide of titanium. 3,001,893 9/1961 Kreuchen 117223,088,921 5/1963 Heischman 252-51 References Cited 5 ST P L. PrimaryExaminer 3,441,516 4/1969 Milligan et a1. 106-49 ESPOSITO,AssistantExaminer 3,394,087 7/1968 Huang et a1. 10648 3,180,841 4/1965Murphy 252 516 2,851,376 9/1958 Adlassnig 106-48 10648, 49; 117-421;252-516 3,503,801 3/1971 Huang et a1. 117-221

